This invention relates generally to aircraft navigation systems and more particularly to an interference suppression circuit for use in airborne VHF Omnirange (VOR) receivers.
In general aviation, the standard navigation system is a VHF Omnirange network, commonly referred to as VOR, wherein the pilot of an aircraft can determine his bearing with reference to the location of selected VOR transmitters. The transmitters at various geographical locations operate on different VHF carrier frequencies, but each conveys a pair of 30-Hz signals which, when received by the aircraft, provide an indication of the angular position of the aircraft relative to the particular transmitter by means of the phase relationship of the signals. Ordinarily, one of the 30 Hz signals is established as a phase reference signal and is frequency modulated upon a subcarrier having a nominal frequency of 9960 Hz, the subcarrier in turn being amplitude modulated upon the VHF carrier. The VOR station also transmits a CW radio signal through a rotating directional antenna such that a 30 Hz AM signal is provided in space at the receiving aircraft. This AM signal has a varying phase for each degree of bearing away from a reference bearing chosen for the station. The reference bearing is due North. An aircraft receiving both the AM and FM signals contains a phase detector which detects the phase difference between the 30 Hz modulation carried by the AM and FM signals. This phase difference is measured in degrees and is a direct indication of the bearing of the aircraft relative to the reference phase of the VOR station.
In order to ensure that accurate bearing indication is obtained, it is essential that the airborne receiver precisely measure the phase relationship between the reference FM signal and the rotating AM signal. Unwanted interference signals occurring close to the frequency of interest, or harmonics thereof, must be filtered to ensure that accurate phase comparisons are made, without slowing the display of bearing information to an unacceptable degree. Prior art VOR receivers, such as the circuit disclosed by Lundgreen et al in U.S. Pat. No. 3,386,096 have employed phase lock loops to achieve this, but this can introduce variable phase errors because of the inherent nature of the devices.